A “coupon reader” is a device that accepts a “coupon,” typically a planar tray having a chemically reactive section that is imbued with a chemical mixture that is designed to react to a target substance and forms a visually detectable indicator pattern after a predetermined development time. A common implementation is the lateral flow immunoassay. The term “ticket” is also sometime used to refer to a coupon. Generally, coupons are designed to be read by a human technician, although coupon readers are also becoming common. The reader automates detection of the indicator pattern, removing reliance on human interpretation. Typically, a single company makes both coupons and the readers for those coupons, but there are also companies that do not make coupons, but do make coupon readers for coupons made by other companies (“third-party coupon readers”). Some coupons have only one section that reacts to a single target substance, whereas other coupons have a number of sections, each one designed to react to a different target substance. Coupons come in various sizes, but it appears that no currently available coupons are greater than 15 cm in any dimension.
Coupons are used to detect substances of interest in the medical and public safety fields. For example, some coupons are designed to accept a bodily fluid onto the chemically reactive sections, to detect a naturally occurring target compound, such as a hormone, or a toxin. A public safety coupon can accept samples from many sources, including aerosolized particulates, liquids, and solids. Air-derived samples, mixed and concentrated into a liquid solution, and placed onto the chemically reactive sections, can provide an indication of an aerosol poison. Typically, the sample is incorporated into a buffer solution that is applied to the coupon, to facilitate the exposure of the coupon chemicals to the atmospheric agent or test specimen. Each chemically reactive section is referred to as a channel. Coupons may have multiple channels, each detecting a different target substance, allowing for multiple assays on a single coupon. Each channel typically includes a control section, which will develop in tandem with the test section of the coupon (the portion of the coupon on which the test line will appear), but unlike the test section will display the indicator pattern, whether or not the target substance is present. The control section performs two functions, first, if the control section does not display the indicator pattern it is generally an indication that something has gone wrong with the process of exposure. Accordingly, a negative reading does not, in that instance, indicate an absence of the target substance, but only indicates a test that was not performed correctly. Also, it shows the test personnel what the indicator pattern looks like.
In many situations, it is important to obtain a quick result from the coupon exposure, but the coupon manufacturer may have designated a set period of time for the indicator pattern to develop. Human technicians may set the coupon aside and attend to other tasks while the coupon develops for the manufacturer-specified time period. Variation in buffer solution and how it is applied can cause the same coupon to develop differently even when exposed to the same concentration of target substance. The use of a reader can lead to a faster detection of the indicator pattern when compared to a human, particularly in low light or stressful situations.
For the purpose of automatically analyzing the visually detectable indicator pattern of the coupon using a camera, it is desirable to provide uniform illumination over the surface of the coupon, as this allows full use of the camera's dynamic range over the entire field of view. As an example, if pixels directly under the light source have a reflected intensity of 255 units while more dimly illuminated pixels at the edge of the coupon have a maximum reflected intensity of 128, then the dynamic range and resolution of a measurement may be lower by a factor of two for assay channels near the coupon edge as compared to channels directly under the light source. In addition, if light rays illuminate a particular part of the coupon surface at oblique angles only, shadows can be created that confound analysis of assay results. These issues are compounded by the fact that a compact portable device is desirable, as it is more easily moved from place to place. Most bioassay coupons are on the order of 11 cm in maximum diagonal measurement, and in the interests of having a compact device, it is desirable that the distance from the coupon to the camera lens is of similar magnitude. To maximize the number of pixels in the analysis, it is also essential that the coupon fill the field of view as much as possible. That is, a near-field macro imaging design is preferred.
Light sources typically used for camera systems are so-called “white LEDs.” Uniform near-field illumination is sometimes accomplished using a circular array of white LEDs surrounding the camera lens, referred to as a ring light. Ring lights may have multiple circular rings of LEDs and may contain more than 100 LEDs. Such a large number of LEDs could significantly reduce battery life and thus operation lifetime in a battery-operated instrument. In addition, commercially-available ring lights still produce an illumination field in which intensity peaks in the center and drops off radially, albeit much more gradually than with a single LED. Since bioassay coupons are typically rectangular, ring lights may not provide uniform edge-to-edge illumination.
Using a single LED light source is far preferable, but the hotspot problem must be solved. The LED cannot be mounted on the same axis as the camera lens: It must be physically located to the side of the lens, and the hotspot consequently appears at an off-axis camera image point. This does not improve the situation since parts of the coupon remain farther away than others from the LED, and light intensity over the coupon surface can be significantly nonuniform across the face of the coupon.
Some improvement in light uniformity can be realized if a piece of ground glass or opalescent glass is mounted directly under the light source. This causes light to be re-emitted in a Lambertian pattern by virtue of the ground glass's surface roughness or by internal refraction in the opalescent glass. That is, the distribution of light emerging from the surface approximates a cosine function relative to a surface normal. These methods more broadly disperse the light, but there still remains a radial distribution of ever-decreasing light intensity relative to an axis passing through the LED source's center point and perpendicular to its surface.
Therefore, there remains a need in the art for improved methods of reading a coupon that, among other things, minimize or compensate for variations in illumination over the surface of the coupon.